Add validation

app/controllers/admin/admin_contacts_controller.rb

@@ -0,0 +1,48 @@
+class Admin::AdminContactsController < ApplicationController
+  before_action :set_domain
+  before_action :set_contact, only: [:destroy]
+
+  def new; end
+
+  def create
+    contact = Contact.find_by(id: params[:contact_id])
+    unless contact
+      flash.now[:alert] = I18n.t('shared.contact_was_not_found')
+      render 'new' and return
+    end
+
+    if @domain.admin_contacts.exists?(contact)
+      flash.now[:alert] = I18n.t('shared.contact_already_exists')
+      render 'new' and return
+    end
+
+    @domain.admin_contacts << contact
+    flash[:notice] = I18n.t('shared.contact_added')
+    redirect_to [:admin, @domain]
+  end
+
+  def destroy
+    unless @domain.can_remove_admin_contact?
+      flash[:alert] = @domain.errors[:admin_contacts].first
+      redirect_to [:admin, @domain] and return
+    end
+
+    if @domain.admin_contacts.delete(@contact)
+      flash[:notice] = I18n.t('shared.contact_detached')
+    else
+      flash[:alert] = I18n.t('shared.failed_to_detach_contact')
+    end
+
+    redirect_to [:admin, @domain]
+  end
+
+  private
+
+  def set_domain
+    @domain = Domain.find(params[:domain_id])
+  end
+
+  def set_contact
+    @contact = Contact.find(params[:id])
+  end
+end